<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .box{
        width: 200px;
        height: 200px;
        margin:50px auto;
        position: relative;
        overflow: hidden;
    }
    .box .forward{
        top:0;
        left:0;
        position: absolute;
        width: 200px;
        height: 200px;
        background-color: aqua;
        transition: all 1.5s;
        transform: rotateY(180deg);
    }
    .box .back{
        position: absolute;
        top:0;
        left:0;
        width: 200px;
        height: 200px;
        background-color: #ff0;
    }
</style>
<body>
    <div class="box">
        <div class="forward"></div>
        <!-- <div class="back"></div> -->
    </div>
</body>
</html>